some stupid convert stuff

src/LazilyInitializedFields.jl

@@ -92,7 +92,6 @@ Base.show(io::IO, b::Box) = show(io, b.x)
 
 
 # This is a replication of the Nothing and Missing conversion functionality from Base.
-# nonuninittype(::Type{T}) where {T} = Core.Compiler.typesubtract(T, Uninitialized)
 if isdefined(Base, :typesplit)
     nonuninittype(::Type{T}) where {T} = Base.typesplit(T, Uninitialized)
 else
@@ -113,9 +112,16 @@ function nonuninittype_checked(T::Type)
     return R
 end
 
+# These are awful
 Base.convert(::Type{T}, x::T) where {T>:Uninitialized} = x
 Base.convert(::Type{T}, x) where {T>:Uninitialized} = convert(nonuninittype_checked(T), x)
+Base.convert(::Type{T}, x) where T>:Union{Uninitialized, Nothing} = convert(nonuninittype_checked(T), x)
+Base.convert(::Type{T}, x) where T>:Union{Uninitialized, Missing} = convert(nonuninittype_checked(T), x)
+Base.convert(::Type{T}, x) where T>:Union{Uninitialized, Missing, Nothing} = convert(nonuninittype_checked(T), x)
+# Ambiguity resolution
 Base.convert(::Type{T}, x::T) where T>:Union{LazilyInitializedFields.Uninitialized, Nothing} = x
+Base.convert(::Type{T}, x::T) where T>:Union{LazilyInitializedFields.Uninitialized, Missing} = x
+Base.convert(::Type{T}, x::T) where T>:Union{Nothing, Missing, LazilyInitializedFields.Uninitialized} = x
 
 
 struct NonLazyFieldException <: Exception
@@ -134,8 +140,8 @@ end
 Base.showerror(io::IO, err::UninitializedFieldException) =
     print(io, "field `", err.s, "` in struct of type `$(err.T)` is not initialized")
 
-@inline _setfield!(x, s::Symbol, v) = ismutable(x) ? setfield!(x, s, v) : getfield(x, s)[] = v
-@inline _getfield(x, s::Symbol) = ismutable(x) ? getfield(x, s) : getfield(x, s)[]
+@inline _setfield!(x, s::Symbol, v) = !isimmutable(x) ? setfield!(x, s, v) : getfield(x, s)[] = v
+@inline _getfield(x, s::Symbol) = !isimmutable(x) ? getfield(x, s) : getfield(x, s)[]
 
 """
     init!(a, s::Symbol)

test/runtests.jl

@@ -59,6 +59,12 @@ end
 end
 box = Boxed(1, uninit)
 
+@lazy struct Boxed
+    a::Int
+    @lazy b::Float64
+end
+box = Boxed(1, uninit)
+
 @testset "Boxed" begin
     @test isimmutable(box)
     @test Boxed(1, 1).b === 1.0 # test conversion constructor
@@ -70,6 +76,27 @@ box = Boxed(1, uninit)
     @test !(@isinit box.b)
 end
 
+@lazy struct BoxedUnion
+    a::Int
+    @lazy b::Union{Missing, Nothing, Int}
+end
+boxed_union = BoxedUnion(1, uninit)
+
+@testset "Boxed" begin
+    @test isimmutable(box)
+    @test !(@isinit box.b)
+    if VERSION >= v"1.2"
+        @test BoxedUnion(1, 2.0).b === 2
+    else
+        @test_broken BoxedUnion(1, 2.0).b === 2
+    end
+    @init! box.b = 2
+    @test @isinit box.b
+    @test box.b == 2
+    @uninit! box.b
+    @test !(@isinit box.b)
+end
+
 @testset "misc" begin
     @test_throws LoadError @macroexpand @lazy a::Int
     @test_throws LoadError @macroexpand @lazy struct Bar end